Photochromism is a property which has been used in the manufacture of light transmissible articles for many years. A compound is said to be photochromic if it changes colour when irradiated and reverts to its original colour when irradiation ceases. The use of photochromics in the manufacture of spectacle lenses is a particular benefit as it enables the efficiency with which radiation is filtered to be varied with the intensity of radiation. Photochromics also have potential for use in a range of other polymeric compositions in products or in applications such as architectural, automotive and aircraft windows and transparencies; inks, paints and coating compositions; optical sensors, filters, switches and data storage devices; and for security and UV exposure sensing applications.
Despite the successful use of photochromic compounds in applications such as lenses there remain limitations which reduce the versatility and potential of this technology. It is advantageous to control the rate at which photochromic polymeric compositions transform to their coloured state when exposed to radiation and fade to clear on cessation of this exposure. In many situations, it is important to provide rapid colouring and fading kinetics particularly for lenses and spectacles. In the past a compromise had to be made in the components and properties of the substrate to enhance the rate of coloration and fade. For example, many photochromics colour and fade more rapidly in soft materials and yet, for applications such as spectacles, abrasion resistance and hardness are important. This trade off between rate of transformation and hardness produces a dilemma for manufacturers between toughness and photochromic efficiency. In polymeric lenses many photochromics exhibit a slower rate of fade than is desirable.
International Application WO2004/041961 describes photochromic adducts which comprise a photochromic moiety and one or more polyether or siloxane oligomers. The presence of the oligomers can significantly improve the rate of coloration and fade particularly in the case of polydialkylsiloxane oligomers. International patent application WO 2009/146509 discloses photochromic polymers which include two or more photochromic moieties linked via a polydialkylsiloxane chain. The polydialkylsiloxane adducts containing a single photochromic show a propensity to phase separate with increased loading in host polymers used for lens materials. Phase separation is reduced for siloxanes substituted with two or more photochromic moieties but we have found that the propensity for phase separation may still be a problem at high loadings. Another problem we have found with siloxane photochromic polymers is their susceptibility to acid degradation which requires special synthetic approaches which are less attractive for industrial manufacture.